At what point does a song become a folk song?

Ms Karpeles wondered about that one too. Ye Olde Dead Hande of 1954 Dogma with regard to this one is:

a) It happens.
b) Except when it doesn't.
c) When it happens, it usually takes quite a long time.
d) But it can be quite quick.

Rifleman - I'm sure that everyone who thinks that a song ceases to be a folksong the moment it's written down will be up in arms. Or they will when they get back from picketing the Bodleian Web site.